The present invention relates to a circuit protection device for use in an electronic apparatus, a battery-driven mobile electronic apparatus, etc.; more specifically, those circuit protection devices used in memory devices such as hard disk drives, optical disk drives built in personal computers or mobile personal computers.
Some of the circuit protection devices (hereinafter referred to as circuit protector) for protecting a circuit board or the like apparatus from an over current have been disclosed in, for example, Japanese Laid-open Patent Publications No. H02-43701, No. H05-120985, No. H03-201504 and so on. Along with the increasing popularity of downsized electronic apparatus, the demand is increasing for components of smaller dimensions. At the same time, requirements in characteristics of such components are becoming more stringent.
A circuit protector disclosed in the Publication No. H02-43701 has an alumina substrate of flat sheet form provided with a nickel layer formed on the upper surface. The nickel layer is trimmed by a laser beam to form a narrowed portion. The electric current concentrates to the narrowed portion, which melts down upon an over current.
In the circuit protectors of the above structure, the heat expected to concentrate to the narrowed portion easily diffuses through the substrate, since alumina substrate has a high thermal conduction coefficient. The heat escapes to wiring of a circuit board through terminals of a circuit protector. As a result, fusing characteristics of a circuit protector tends to fluctuate depending on various conditions such as wiring arrangement in the relevant circuit board, etc.
Thus, the conventional circuit protectors are not capable of controlling the heat diffusion to a circuit board effectively. As a result, the pre-arcing time-current characteristics (hereinafter referred to as xe2x80x9ccharacteristicsxe2x80x9d) and other performance items remain out of a stringent control.
A circuit protector disclosed in the Publication No. H05-120985 has a pair of conductive portions provided on an insulating substrate, and a fuse member is formed between the pair of conductive portions. The fuse member is covered with a JCR coating, which is further covered with a resin mold.
The above-described structure is complicated and needs an increased number of process steps. The problem on top of it is that dispersion in the characteristics is relatively wide.
Further, the Publication No. H03-201504 discloses a fuse resistor provided with a melt down portion between two terminals. The melt down portion is made by narrowing a resistor film by two end portions of not continuous grooves.
The structure has a problem of wide deviation of characteristics, or pre-arcing times.
A circuit protector of the present invention comprises a substrate, a conductive layer formed around the substrate, a narrowed portion formed on the conductive layer at a certain part, terminals formed at both ends of the substrate. The substrate has 1-30% pores in a unit surface area in a vicinity of its surface. The present invention also relates to a structure relating to the mounting circuit protectors on a circuit board.